The present invention relates to a method and apparatus for cleaning filter elements of a filter apparatus. More particularly, it relates to a method and apparatus for cleaning filter elements provided with a filtering material upon a surface of which a cake is deposited during a filtering process, particularly in a centrifugal filter after alluvial filtration of coal in flowable state.
Alluvial filters have been used for filtration of coal in flowable state. Such a filter has a hollow shaft and a plurality of horizontal plates mounted on the shafts and spaced from one another in direction of elongation thereof. The hollow shaft and the plates are accommodated in a pressure vessel. The operation of this filter is generally satisfactory. In dependence upon the origin of the coal, as well as upon the utilized filter aids, a cake is deposited during a filtering process on the filtering elements and forms a mass which is more or less difficult to be removed. In many cases the complete removal of this mass is so difficult that in short time the filter must be opened and the filtering material of the filter elements must be mechanically or hydraulically cleaned.
It has been known to utilize spraying nozzles in a centrifugal filter apparatus for removal of a filter cake. In the Swiss patent 333,492 a filter apparatus is disclosed, in which nozzle elements supply fluid medium for cleaning the filter elements. In accordance with this patent, the filter cake is washed off by the fluid medium. In accordance with a method disclosed in the Swiss patent 402,821 a solvent in a vapor phase is pressed through the filter cake. A liquid is produced during partial condensation of the vapors in the interior of the cake and washes away the cake from inside. The solvent in the vapor phase is supplied from outside of the filter vessel or the solvent is vaporized inside the filter. An arrangement for vaporization of the solvent is not disclosed.
A disadvantage of the above-mentioned method of cleaning by means of fluid medium is that it always requires very much solvent for washing away the filter cake. Moreover, by spraying the fluid toward the cake the latter is only being eroded but not removed, and the filtering material of the filter elements is not cleaned. Generation of solvent vapors inside or outside the filter vessel requires expensive arrangements and is not suitable for removal of the filter cake and cleaning of the filter material.